1. Field of the Invention
The present invention relates to a vehicle suspension apparatus and, more particularly, to a suspension apparatus which effectively cancels a vibration input of a predetermined frequency input from a road surface.
2. Description of the Related Art
An active suspension apparatus has been proposed for improving driving stability and comfortability. Since the active suspension costs too much, instead of the active suspension, what is called a semi-active suspension apparatus has been proposed. In the semi-active suspension apparatus, a hydraulic device suppresses the displacement of posture of a vehicle body in a low-frequency band, and a shock absorber (damper) damps posture displacement in a high-frequency band.
Japanese Patent Application Laid-Open No. 60-248419 and Japanese Patent Application Laid-Open No. 3-72415 disclose a semi-active suspension apparatus comprising a shock absorber (damping device) between sprung mass on the vehicle body side and unsprung mass on the wheel side for damping vertical vibration of the wheels. The shock absorber has variations such as an absorber which changes, in a stepwise manner, a damping coefficient representing a damping force in two steps (large/small), an absorber which changes the damper coefficient in a multi-stepwise manner, and an absorber which changes the damper coefficient in a continuously-stepless manner.
The basic control method for the damper-coefficient variable shock absorber is as follows. The damper coefficient is changed so that damping force generated by the shock absorber becomes the target damper force (what is called "a sky-hook damper force") for preventing displacement of the sprung mass in a vertical direction. Japanese Patent Application Laid-Open No. 60-248419 discloses examining a positive/negative sign of the difference between the sprung mass position and the unsprung mass position and a positive/negative sign of the difference between the sprung mass velocity and the unsprung mass velocity to determine whether or not the signs coincide with each other. If the signs coincide with each other, the shock absorber damper coefficient is increased to raise the damper force; if not, the shock absorber damper coefficient is decreased to lower the damper force.
FIG. 1 shows frequency characteristics of an acceleration signal of the sprung/unsprung mass with respect to road surface input signals in the conventional semi-active suspension apparatus model.
In FIG. 1, a solid line indicates the frequency characteristic when the setting of the shock absorber is "hard"; a dot-dash line, the frequency characteristic when the setting is "soft"; a broken line, the frequency characteristic when the setting is "normal". As shown in FIG. 1, when the shock absorber setting is normal, an acceleration peak appears at around 4-7 Hz, and at this time, a driver feels "jolted". A peak at around 10-12 Hz causes the driver to feel "trembling" or "rattling". If the damping setting is soft, as represented by the dot-dash line the "jolted" and "trembling" feelings are augmented/increased. On the other hand, if the damping setting is hard, the acceleration characteristic exhibits high value from a low-frequency to a high-frequency with a peak at around 10-12 Hz, as represented by the solid line. Changes of acceleration do not appear in both the soft damping and normal settings but appear in the hard damping setting causing the driver to feel "vibrating".
To improve the suspension characteristics in all the frequency bands, hard-setting of the shock absorber damping with respect to a road surface vibration input in the low-frequency band (the hydraulic device is employed for the low-frequency input) and soft-setting of the damping with respect to a road surface vibration input in the high-frequency band (in consideration of the costs when the expensive full-active suspension is used for high-speed response to the high-frequency vibration) can be proposed. For example, in FIG. 1, the damping with respect to the frequency bands I and III may be set as "normal" and the damping with respect to the frequency band II may be set as "hard".
However, this arrangement cannot prevent comparatively high acceleration in the frequency band II, therefore, further prevention is needed. In addition, the phenomenon that the driver feels "trembling" with respect to the road surface vibration input in the 10-12 Hz frequency band also occurs in the full-active suspension apparatus as shown in FIG. 2.
Since a vibration input changes transitionally, prediction of such vibration inputs is difficult. For this reason, there has been no effective solution for the resonance with respect to a vibration input of a specific frequency band. Regardless of full-active suspension or semi-active suspension, the conventional suspension apparatuses have mainly performed feedback control, in which the hydraulic control and shock absorber control are made based on a transitionally variable road surface vibration. However, a time delay occurred in this feedback control rather induces resonance.